In Japanese laid-open application No. 63-104618 of Kawada et al. there is disclosed a method of producing hollow fiber composite membranes. The method shown involves continuously casting a thin film on water and passing microporous hollow fiber through the polymer solution in a region where the polymer/solvent solution possesses fluidity. The solution deposits a thin film on the membrane while excess polymer is taken up and stored. The system utilized by Kawada et al. does not address the issues of depositing a thin film uniformly about the periphery of a hollow fiber as would be required for high quality separation membranes. Moreover, significant control and recycling of polymer would be required, making the system difficult to automate or even produce commercially suitable product.
A superior method of coating fibers is disclosed and claimed in U.S. Ser. No. 07/582,691, noted above, which involves providing a polymer solution to the surface of a liquid bath to form a polymeric film and drawing the fiber therethrough while radially advancing the film to a central point. Especially advantageous operation is achieved by draining the bath at the central point as will be appreciated by reference to the discussion which follows. In processes such as the foregoing, however, it is difficult to separate the fiber from liquid with which it has been associated during processing, and in many cases excess liquid must be evaporated or simply tolerated regardless of the specific processing method employed and may lead to inferior product.